Electrical resistance heaters suitable for heating subterranean earth formations have been under development for many years. These heaters have been found to be useful for carbonizing hydrocarbon-containing zones for use as electrodes within reservoir formations, for enhanced oil recovery and for recovery of hydrocarbons from oil shales. U.S. Pat. No. 2,732,195 discloses a process to create electrodes utilizing a subterranean heater. The heater utilized is capable of heating an interval of 20 to 30 meters within subterranean oil shales to temperatures of 500.degree. C. to 1000.degree. C. Iron or chromium alloy resistors are utilized as the core heating element. These heating elements have a high resistance and relatively large voltage is required for the heater to extend over a long interval with a reasonable heat flux.
Subterranean heaters having copper core heating elements are disclosed in U.S. Pat. No. 4,570,715. This core has a low resistance, which permits heating long intervals of subterranean earth with a reasonable voltage across the elements. Because copper is a malleable material, this heater is much more economical to fabricate than iron or chromium alloy cored heaters. These heaters can heat 1000-foot intervals of earth formations to temperatures of 600.degree. C. to 1000.degree. C. with 100 to 200 watts per foot of heating capacity with a 1200 volt power source. They could therefore be useful in thermal recovery of hydrocarbons from heavy oil reservoirs and from oil shales.
The capital investment required to utilize these heaters to recover hydrocarbon from subterranean formations generally renders the use of such heaters economically unviable. These heaters each require casings within the well borehole to protect the heaters. The casings themselves must be capable of withstanding 600.degree. to 1000.degree. C. temperatures in corrosive environments. The heaters are suspended within the casings in a gas environment. The casing therefore does not have a significant hydrostatic head on the inside. The casing is therefore generally exposed to high crushing forces. High crushing forces dictate that the casing be of significant thickness. Casings for wells utilizing these heaters therefore represent a major investment.
It is therefore an object of the present invention to provide a subterranean heater which does not require a casing.
It is another object to provide a subterranean heater which can provide from about 100 to about 200 watts of heat per foot of heater length for a 20-year or more useful life.
In another aspect, it is an object of the present invention to provide a process to heat subterranean formations which do not require casings in heat injection wells.